1. Field of the Invention
The present invention generally relates to packet data wireless communications systems and, more particularly, to continuing packet data communications through one or more neighboring mobile units until a new connection is established in the new cell.
2. Background Description
With the advent of in-building packet data wireless data communication standards, portable personal computing devices such as laptop computers, PDAs (Personal Digital Assistants), palmtop computers, and the like will be used to access data through wireless communication links. In such systems, while wireless links eliminate cables and provide tetherless access, full mobility across the wireless network coverage area is dependent on) the ability of the systems to provide seamless handoffs between access points. However, in practical systems, the number of mobile units (slaves) that can be actively connected to a base station or an access point (master) of a cell is limited to a certain number dependent on the system. For example, in one standard, a maximum of seven slaves can be actively linked to a single master at any time. Therefore, when a mobile unit moves to a new cell, it could experience indefinite interruption in communication if the new base station is already busy with all the allowed maximum number of connections. But in practice, it is desirable to maintain communication for the handoff mobile unit even though at a reduced data transfer rate.
Handoff methods which involve the assistance of mobile units have been proposed. See for example U.S. Pat. No. 5,042,082 to Dahlin for xe2x80x9cMobile Assisted Handoffxe2x80x9d, U.S. Pat. No. 5,634,192 to Meche et al. for xe2x80x9cMobile-Assisted Handoff Techniquexe2x80x9d, and U.S. Pat. No. 5,200,957 to Dahlin for xe2x80x9cMobile Assisted Handoffxe2x80x9d. The role of mobile units in such handoff methods is mainly to provide to base station(s) information such as received power levels and candidate base stations for handoff.
Handoff schemes such as that proposed in U.S. Pat. No. 5,175,867 to Wejke et al. for xe2x80x9cNeighbor-Assisted Handoff in a Cellular Communication Systemxe2x80x9d involve the participation of neighboring base stations. U.S. Pat. No. 5,640,679 to Lundqvist et al. for xe2x80x9cMethod and Apparatus for Handing Off Mobile Station From a First to a Second Channel in a Mobile Communication Systemxe2x80x9d describes a procedure for handing off a mobile unit in a time division multiplex (TDM) channel sharing system wherein a time offset to be used in the new cell is determined before the handoff is effected.
One of the goals of handoff techniques is to provide handoff without interruption in communication. Keeping this in mind, some methods, such as that presented in U.S. Pat. No. 5,483,668 to Malkamaki et al. for xe2x80x9cMethod and Apparatus Providing Handoff of a Mobile Station Between Base Stations Using Parallel Communication Links Established with Different Time Slotsxe2x80x9d, establish parallel communication links with the next base station to which a handoff is expected.
While existing handoff schemes perform well if the next base station has free channel resources, there is still a need to address the problem of handoff to a busy base station. In a situation wherein all the neighboring base stations are busy to their full capacity, and if a mobile unit moves to one of the busy cells, it has to wait until channel resources become available in the new cell. During this indefinite waiting period, it is desirable to provide data communication to the mobile unit through alternative means.
It is therefore an object of the present invention to provide continuing data communication during handoff waiting periods by utilizing neighboring mobile units, if any, to forward data packets destined for the handoff mobile unit.
It is another object of the invention to maintain communication for a handoff mobile unit even though at a reduced data transfer rate.
According to the invention, communication for a handoff mobile unit is maintained even when the master unit has no available channels. In this case, the mobile unit switches over to master status to create an ad hoc cell. The mobile unit then pages neighboring slave mobile units for data forwarding assistance. The mobile unit chooses one or more slave mobile units and establishes wireless links with the chosen slave mobile units. These chosen mobile units then have two masters; one the master unit of the cell and the other the handoff mobile unit which has created the ad hoc cell. A list of the slave mobile units in the ad hoc cell is sent to the network server, and data communication proceeds with data forwarding through the chosen mobile slave units. When a channel becomes available with the master unit for the cell, the handoff mobile unit becomes a slave mobile unit to the master unit and the ad hoc cell is dissolved.